The basic directional coupler is a linear, passive, four port network, incorporating two parallel coupled transmission lines. A first transmission line extends between an input port and a through port, and a second transmission line extends between a coupled port and an isolated port. A signal applied to the input port propagates along the first transmission line and induces a coupled signal into the second transmission line. In so-called backward-wave couplers, the coupled signal propagates in the reverse direction with reference to the transmission line to which the input signal is applied.
A fundamental TEM directional coupler is shown in the textbook “Microwave Filters, Impedance-Matching Networks, and Coupling Structures” by Matthaei et al., McGraw Hill, Chapter 13. A directional coupler with broadside coupled striplines is described in the article “Characteristic Impedance of Broadside-Coupled Strip Transmission Lines” by S. Cohn, IRE MTT, November 1960. A directional coupler with offset broadside coupled lines is described in the article “Impedances of Offset Parallel-Coupled Strip transmission Lines” by J. P. Shelton, Jr., IEEE MTT, Vol. MTT-14, No.1, January 1966. Another directional coupler is known for example from U.S. Pat. No. 5,570,069. All these documents are herewith incorporated by reference herein.
The prescribed spatial relationship of the coupled lines in a directional coupler with broadside coupled striplines must be accurate in order to achieve the desired electrical response. In such strongly coupled lines, the gap between the lines is often very small compared to the width of the coupled lines and variations must be kept to a tolerable minimum. At the same time any metallic or non-metallic adjustment means for the coupled lines interfere with the electromagnetic fields around the lines and thereby become themselves a source of performance degradation.
It is an object of the present invention to provide a directional coupler with improved characteristics and increased production yield.